1) Field of the Invention
This invention relates to a single-mode optical fiber that has a core and a cladding.
2) Description of the Related Art
There is a demand for more inexpensive ways to provide communication services, at various speeds, as a social infrastructure supporting the recent advanced information society, which is typified by Internet services. The introduction of optical fiber for offices and homes or apartments (FTTH: Fiber To The Home) is advancing at a rapid speed. With respect to optical fiber used for FTTH, just as with metal cables, ease of handling is required. That is, it is required that surplus cables are storable compactly in closures and cabinets; and that they are not damaged in the case of a momentary bend being added, such as hooking. If such demand is taken into consideration, the conventional single-mode fiber (SMF) specified in International Telecommunication Union Telecommunication Standard Sector (ITU-T) G.652 is unsuitable, since a large macro bending loss arises.
Generally, in the case of adding a bend to an optical fiber by external stress, the transmission loss of the optical fiber becomes large. This increase of transmission loss, due to a bend, is called a macro bending loss. The smaller the bending radius is, or the longer the wavelength is, the greater the macro bending loss arises. And the increase becomes exponential. In ITU-T, the wavelength region of 1260 nm to 1625 nm is defined as the transmission wavelength region of optical fiber used for Passive Optical Network (PON) systems, and optical communications in the same wavelength region are performed with FTTH. Additionally, if a wavelength band of monitoring light is to be added to this wavelength region, it is desirable that the optical fiber transmits optical signals excellently in a wavelength up to 1650 nm at the longest. In other words, the macro bending loss of an optical fiber used for FTTH needs to be sufficiently low, even when it transmits light signals at a wavelength of 1650 nm.
As an optical fiber that addresses these points, the optical fiber having a trench type refractive index profile in which the cladding area is composed of the cladding and an area with a lower refractive index than the cladding, is known (for example, see Fujikura Ltd., Optics and Electronics Laboratory, Optical process research department, Masashige Ikeda, Shoichiro Matsuo, Kuniharu Himeno, “Reduced Splice Loss Type Low Bending Loss Optical Fiber”, The institute of electronics, information and communication engineers, Technical report of the institute of electronics, information and communication engineers, OCS2003-43, OFT2003-25 (2003-8)).
And, when an optical fiber cable is to be laid for users' homes at the introduction of a FTTH system, it is expected that bends will be added to a drop cable, to an extent equivalent to adding 10 turns at a bending radius of 7.5 mm, between a point the drop cable is branched from a main line up to the connection to the ONU (Optical Network Unit).
On the other hand, for example, in a conventional SMF complying with ITU-T G.652, when adding 10 turns of bends of bending radius of 7.5 mm, a macro bending loss arises of about 40 dB at a wavelength of 1550 nm, and approximately 120 dB at a wavelength of 1650 nm. Additionally, with the optical fiber presented in the document cited above, the reduction of a macro bending loss at 1650 nm is still insufficient. Given this situation, reducing the macro bending loss of optical fiber is the basic technology critical for spread of FTTH. If the macro bending loss is 0.1 dB or less, in the case of adding 10 turns of bends of bending radius of 7.5 mm, the macro bending loss is lowenough to apply for FTTH systems.
With a conventional SMF made of silica glass, there is a need to make the effective refractive index of the fundamental mode high to reduce the macro bending loss. The high effective refractive index is realized by making relative refractive difference Δ between the core and the cladding large in a refractive index profile of the optical fiber. The relative refractive index difference Δ is defined by the following equation (1). In the equation (1), ncore, and ncladding are the refractive indexes of the core area and cladding area respectively.Δ={(ncore−ncladding)/ncore}×100%  (1)
FIG. 1 shows the results obtained by simulations of the relations among the relative refractive index difference Δ, the macro bending loss of bending radius of 7.5 mm at the wavelength of 1650 nm, and the mode field diameter (MFD) at the wavelength of 1310 nm, in the case of an optical fiber having a step type refractive index profile. In this simulation, the fiber cutoff wavelength λc was set to 1280 nm. In this case, the cable cutoff wavelength λcc becomes 1260 nm or less.
On this specification, the fiber cutoff wavelength λc and the cable cutoff wavelength λcc are assumed to be the fiber cutoff wavelength λc and the cable cutoff wavelength λcc, which are specified in ITU-T G.650.1. Other terms, which are not specifically defined, are assumed to follow the definition and measuring method on ITU-T G.650.1.
With regard to the relative refractive index difference Δ, when the relative refractive index difference Δ is set high as 0.7% or more, the macro bending loss at the wavelength of 1650 nm becomes low sufficiently as 0.1 dB/10 turns or under, then it is usable for FTTH systems.
On the other hand, the MFD becomes 6.3 μm or under at 1310 nm, i.e., smaller by 3 μm or more than the MFD of conventional SMF. Generally, if the relative refractive index difference Δ becomes larger, the confinement of the guided mode tends to be stronger, and the MFD tends to be smaller.
And, on the introduction of FTTH, at the time of actually laying the optical fibers, there is a need to splice them with the existing laid conventional SMFs. As many fiber installers execute the splicings frequently, it is desirable that the splicing is easy and cost effective, and the splicing loss is low.
The splicing loss T, which arises on splicing optical fiber, is determined by coupling coefficient η. And the splicing loss T is logically calculated by the following equations (2)˜(4):T=−10×log η  (2)η=κ×exp{−κ[(1/w12+1/w22)x02/2]}  (3)κ=4/{(w1/w2+w2/w1)2+(λz/πw1w2)2}  (4)
Here, w1 and w2 are the mode field radiuses of the both optical fibers connected each other, x0 is the lateral misalignment of the fibers, κ is related to the used wavelength, and z represents the separation distance between the optical fiber ends. Here, it is assumed that the fiber axes coincide with each other. A relation between MFD and the splicing loss obtained by the above equations is shown in FIG. 2. In these calculations, providing connecting a conventional SMF having MFD=9.3 μm and a fiber having a various MFDs. The calculation is executed by assuming the separation distance between the ends of the splicing optical fibers z as 0. As can be seen from FIG. 2, when the difference in MFDs between the spliced optical fibers is larger, a larger splicing loss arises
In FIG. 2, it can be seen that such a splicing loss as large as about 0.7 dB arises in splicing of a conventional SMF having MFD of about 9.3 μm at wavelength 1310 nm and an optical fiber having a step type refractive index profile of the relative refractive index difference Δ=0.7% and MFD=6.3 μm. It is desirable for the splicing loss on building of FTTH systems to become 0.5 dB or less. And, on an actual splicing, the fiber axes do not coincide completely because of the dimension of the mechanical splice and the dimensional precision of cladding diameter of optical fiber so it is necessary to take the decrease of the coupling coefficient into consideration for estimating the splicing loss. Considering the worst case, a design of an optical fiber of which the splicing loss with the conventional SMF is sufficiently low under the condition that the lateral misalignment is 0.5 μm is needed. And, the MFD needs to be 6.6 μm or more to make the splicing loss 0.5 dB or less.
As above, in the design of the refractive index profile of optical fiber, if the relative refractive index difference Δ is made larger to reduce a macro bending loss, MFD becomes smaller, and it causes an increase in the splicing loss. A relation between the macro bending loss and the splicing loss is shown in FIG. 3 in the case of changing the relative refractive index difference Δ of an optical fiber having a step type refractive index profile. Here, the horizontal axis represents the splicing loss in the case of splicing a conventional SMF having MFD=9.3 μm at a wavelength of 1310 nm and a fiber having MFD that differs from it. And the vertical axis represents the macro bending loss arising in the case of bending a fiber having various MFD and showing the displayed splicing loss at a bending radius=7.5 mm at a wavelength of 1650 nm. As shown in FIG. 3, a trade-off relation exists between the macro bending loss and the splicing loss.
The result obtained by simulation of the relation between the MFD at the wavelength of 1310 nm and the macro bending loss at the wavelength of 1650 nm is shown in FIG. 4, while setting the fiber cutoff wavelength λc as 1280 nm, and changing the α value represents the shape of refractive index profile with an optical fiber having a step type refractive index profile. The α value is defined by the following equation (5). The center shape of the first layer (core) has a more roundness (shifting triangular shape to quadrangular shape) as the α value becomes larger.n2(r)=ncore2{1−2(Δ/100)×(2r/a)α}  (5)(In this regard, 0<r<a/2)
Here, r represents a position from the center of an optical fiber toward radius, and n(r) represents a refractive index at a position r. As shown in FIG. 4, with a step type refractive index profile, it is found that the relation between the macro bending loss and the MFD is not influenced by the α value, under the above condition, that the fiber cutoff wavelength λc is fixed at 1280 nm and the relative refractive index difference Δ and the α value areadjusted. Then, it is found that the trade-off relation between the macro bending loss and the splicing loss cannot be improved, if the relative refractive index difference Δ and the α value are changed. This means that the step type refractive index profile cannot satisfy the aimed value of the macro bending loss of 0.1 dB/10 turns or less and the splicing loss of 0.5 dB or less.